Single bore tube gas laser

ABSTRACT

A gas laser employing a single bore tube having the laser mirrors attached to and terminating the opposite ends thereof and a bulb surrounding the bore tube, with the ends of the bulb being attached to the bore tube at two spaced points intermediate the ends of the bore tube. A hole through the wall of the bore tube provides communication between the bore and the inside of the bulb. Also disclosed is means for shock mounting such a discharge tube in a cylindrical container with the axis of the bore tube in substantially coincident relationship with the axis of the container.

States aten 1 EEC Mark June 12, 1973 1 SINGLE BORE TUBE GAS LASER [75]Inventor: John Thomas Mark, Lancaster, Pa.

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: June 21, 1971 [21] Appl. No.: 154,826

[51] Int. Cl. H01s 3/02, HOls 3/22 [58] Field of Search 331/94.5;313/217,

[56] References Cited UNITED STATES PATENTS 3,566,302 2/1971 Rhodes331/945 3,619,811 11/1971 Kaiser et a1 331/945 3,627,429 8/1968 Jaenicke331/945 3,298,894 1/1967 Barnette 331/945 3,617,926 11/1971 Bullinger331/945 OTHER PUBLICATIONS Bridges et a1., Laser Focus, Vol. 5, No. 19,Oct. 1969,

Primary ExaminerRonald L. Wibert Assistant ExaminerR. J. WebsterAtt0mey-Edward J. Norton [57] ABSTRACT A gas laser employing a singlebore tube having the laser mirrors attached to and terminating theopposite ends thereof and a bulb surrounding the bore tube, with theends of the bulb being attached to the bore tube at two spaced pointsintermediate the ends of the bore tube. A hole through the wall of thebore tube provides communication between the bore and the inside of thebulb.

Also disclosed is means for shock mounting such a discharge tube in acylindrical container with the axis of the bore tube in substantiallycoincident relationship with the axis of the container.

6 Claims, 3 Drawing Figures 1 SINGLE BORE TUBE GAS LASER This inventionrelates to gas lasers, and, more particularly, to gas lasers having animproved structure which provides increased mechanical and thermalstability.

Relatively inexpensive, low power, sealed gas-lasers, (such as He-Nelasers), incorporating an integralmirror optical cavity, now exist inthe prior art. It is desirable that such lasers have simple, compactstructures in order to achieve both low cost and easy handling thereof.For this reason, laser devices have been developed which have coaxialsymmetry with a bulb of the laser device enclosing and surroundingrespective portions of first and second separate, spaced,axiallyoriented bore tubes having bores which communicate with theinside of the bulb. The first bore tube, after passing through one endof the bulb, is terminated at its distal end by one of the mirrors ofthe optical cavity and the second bore tube, after passing through theother end of the bulb, is terminated at its distal end by the other ofthe mirrors of the optical cavity. One of the electrodes of the laser,such as its cathode, is located within the bulb but outside of eitherbore tube, while the other electrode of the laser, such as its anode, isin cooperative relationship with the bore of one of the first and secondbore tubes, but not the other. Both the bore tubes and the bulb arefilled with laser gas, so that the bulb provides the large reservoir oflaser gas for the bore tubes which is needed to provide sealed gaslasers with long life. By arranging this required reservoir of gas sothat it surrounds a significant portion of both bore tubes, an overallcompact design is achieved for this type of prior art gas laser.

The prior art use of two separate bore tubes, each of which is supportedin cantilever suspension by only that one of the respective ends of thebulb that it passes through, creates problems because it is difficultinitially to align them with the required precision and because they donot maintain their alignment due to the relative mechanical and thermalinstability of such a gas laser. In particular, the alignment of theoptical cavity of the laser is affected by mechanical and thermalstresses to which the bore tubes are subject, so that the power outputof such a laser is not as stable as desirable. This is becausevariations in the ambient temperature of the environment, theorientation of the laser with respect to the vertical and otherenvironmental factors, which either affect the thermal or mechanicalequilibrium of the bore tubes, causes a shift in the power output ofsuch a laser.

The present invention overcomes these problems by employing a singlebore tube, which extends the entire length of the laser and isterminated at its opposite ends by the respective mirrors of the opticalcavity of the laser. This single bore tube, which is preferably made ofa thick-walled capillary tube, is surrounded over an intermediateportion thereof with a bulb, which is preferably disposed coaxially withrespect thereto. A hole through the wall of the bore tube, locatedwithin the intermediate portion of the bore tube, permits communicationbetween the bore and the inside of the bulb. The use of a single boretube, supported at two spaced points by the bulb and surrounded by lasergas provides simply and inexpensively a more rigid tube having desiredmechanical and thermal stability; features absent in prior art lasertubes of this type.

These and other features and advantages of the present invention willbecome more apparent from the following detailed description takentogether with the accompanying drawings, in which:

FIG. 1 is an elevation cross-sectional view of a preferred embodiment ofthe laser tube of the present invention situated in a preferredenvironment,

FIG. 2 is a cross-section of the apparatus shown in FIG. 1, taken alongsection line 2 2, and

FIG. 3 is a fragmentary view showing how a portion of the laser tube ofFIG. 1 is fabricated.

Referring now to FIGS. 1 and 2, there is shown an assembled laser tube10 which, although in no way essential to the invention, is whollycontained within laser head 12. Also shown are various elements, none ofwhich are essential to the present invention, coupled to laser tube 10and/or laser head 12. All of these elements and laser head 12 areincluded in FIG. 1 merely to show a preferred environment for locatinglaser tube 10 which happens to be actually used in practice.

Laser tube 10 itself consists of a single thick-walled capillary tube 14having a bore 16 extending therethrough. The term thick-walled capillarytube, as used herein, means a capillary tube in which the thickness ofthe wall of the capillary tube is at least the diameter of the boretherethrough. For instance, in a practical example capillary, tube 14had an outer diameter of 0.420 inches, while the diameter of bore 16 was0.060 inches. Therefore, in this case, the thickness of the wall ofcapillary tube 14 was 0.180 inches, or three times the diameter of bore16. Thus, a thick-walled capillary tube, such as tube 14, is quiterigid.

As shown in FIG. 1, both left end 18 and right end 20 of capillary tube14 are somewhat concave in shape. In practice, the radius of curvatureof ends 18 and 20 is much less than is shown in FIG. 1. For instance, inthe above example, where the outer diameter of capillary tube 14 is0.420 inches, the radius of curvature of concave surfaces 18 and 20 isthree inches. However, for the sake of clarity in the drawing, theradius of curvature of each of ends 18 and 20 has been exaggerated inthe drawing. Concave ends 18 and 20, which are ground to be sphericalwith great precision (tolerances preferably in the order of only a fewten thousandths of an inch) are used for the purpose of initiallyaligning totaling reflecting mirror 22 and output mirror 24 to be inparallel relationship with respect to each other (i.e. the relationshiprequired for a laser optical cavity). Mirrors 22 and 24, after beingproperly aligned, are permanently cemented to ends 18 and 20,respectively, by a sealant (not shown).

Surrounding a portion of capillary tube 14 is bulb 26, which includes aleft end section 28 and a right end section 30 both of which are fusedto the outside of the wall of capillary tube 14. In the case where theouter diameter of capillary tube 14 is 0.420 inches, the outer diameterof bulb 26 may be about 1 Va inches.

Left end portion 28 includes a plurality of circumferentially disposedpins, such as pins 32 and 34, which extend therethrough. For instance,the total number of such pins may be seven and they may be disposed atequal angular intervals around the circumference of left end portion 28.At least one of the pins, such as pin 32, is connected to cold cathode36, which may be an aluminum cylinder. The one or more pins, such as pin32, which are attached to cathode 36 provide mechanical supporttherefor. In addition, a single one of these pins, such as pin 32itself, is utilized as an electrode to provide electrical connection tocathode 36 from outside laser tube 10. Also providing mechanical supportfor cathode 36 are three spring clips 38, which are distributedcircumferentially at 120 intervals between cathode 36 and the inside ofthe wall of bulb 26.

A pin, such as pin 34, may be employed for purposes as supporting agetter, applying a discharge starting potential, etc. or it may not beutilized at all. Right end portion 30 of bulb 26 includes sealed tip 40from which originated the exhaust tubulation 42, shown in FIG. 3,utilized during the fabrication of the tube. Shield 44 is coupled toright end portion 30 of bulb 26 and is situated as shown in spacedsurrounding relationship with capillary tube 14.

The manner in which shield 44 is connected both to bulb 26 and endportion 30 during fabrication of laser tube is shown in FIG. 3. Theseelements are fused to each other by means of heat supplied by a gasflame 46.

Bore 16 of capillary tube 14hole connected to the interior of bulb 26 bya hold 48 through the wall of capillary tube 14. As shown, hole 48 issituated at the distal end of shield 44 in the vicinity of right endportion 30 of bulb 26.

An anode region 50 communicating with bore 16 is formed by a closed holein the wall of capillary tube 14. Anode region 50 is disposed in arelatively long portion of capillary tube 14 which extends between theleft end portion 28 of bulb 26 and the left end 18 of capillary tube 14itself, as shown. Connecting anode region 50 to the outside of tube 10is electrode pin 52.

It should be understood that tube 10, as just described, may be placedin any appropriate environment. However, in the preferred environmentshown in FIG. 1, tube 10 is wholly contained within laser head 12, whichis composed of a cylinder of metal, such as aluminum. In particular,tube 10 is shock mounted to container 12, in a manner to be describedbelow, by means of elastomer adhesive pads 54, such as silicone rubber,which preferably exhibit a small degree of mechanical hysteresis. Laserhead 12 includes a left end portion 56 in the vicinity of totallyreflecting mirror 22 and a right end portion 58 in the vicinity ofoutput mirror 24. As shown, left end portion 56 is totally enclosed(with the possible exception of ventilation holes therein) to provideprotection for laser 10 at that end. However, right end portion 58 issupplied with threaded central hole 60 to permit output radiation frommirror 24 to emerge from laser 10. During shipment of the complete laserhead including laser tube 10, a screw cap, not shown, may be insertedinto threaded central hole 60 to protect mirror 24. Further auxiliaryequipment, such as a telescope, may be inserted into threaded centralhole 60.

For the purpose of electrically connecting laser tube 10 to a powersupply, not shown, a pair of wires 62 enters laser head 12 throughgrommet 64. One wire 66 of wire pair 62, which is normally electricallygrounded, is connected to head 12 by screw terminal 68. Cathode 36 oftube 10 is therefore also electrically grounded by connection to head 12by means of pin 32, wire 70 and screw terminal 72, as shown. Anode pin52 is connected to the hot side of the power supply by means ofcurrent-limiting resistance 74 and wire 76 of wire pair 62. Inparticular, as shown, current-limiting resistance 74 includes a topportion having a right-end lead which is electrically connected to anodepin 52 by crimped splice 76. Elastomer 78 is used to insulate thisconnection. The left end of the lower portion of resistance 74 isconnected to wire 76 by a spot weld 80 which is insulated by elastomer82. The upper and lower portions of resistance 74 are electricallyconnected in series with each other by a stiff, electrically connectingwire 84. Wire 84, by partially surrounding tube 14, also mechanicallysupports resistance 74. Holding wire 84 around tube 14 is ring 86. Whena suitable excitation voltage is applied to wire pair 62, a gasdischarge is initiated in a laser gas, such as He-Ne that fills bothbulb 26 and bore 16. This discharge extends from anode pin 52, throughanode region 50, bore 16, hole 48, the region between shield 44 andcapillary tube 14, and Finally to cylindrical cathode 36. The purpose ofshield 44 is to guide the gas discharge plasma and spread it over arelatively large area of cathode 36. This prevents any given local pointof cathode 36 from experiencing an electrical field high enough toresult in undesirable sputtering of the cathode material. By preventingsputtering, the presence of shield 44 lengthens the life of laser tube10.

Since only bore 16 is in cooperative relationship with mirrors 22 and24, only the portion of the discharge which takes place in bore 16 is incooperative relation ship with the optical cavity formed by mirrors 22and 24. Therefore, lasing takes place only in bore 16, with theresulting laser output radiation being transmitted by partiallyreflecting output mirror 24.

Peak power output of the laser depends upon mirrors 22 and 24 beinginitially placed in strict parallel relationship with respect to eachother and then maintaining this relationship. Furthermore, it isdesirable that bore 16 be substantially coincident with the axis oflaser head cylinder 12 independent of the angular orientation thereof.Since bulb 26 is made of relatively cheap glass tubing, in order toachieve peak power from the laser, it is important that the exactposition of laser tube 10 within the laser head 12 be established priorto the application of elastomer pads 54. In order to accomplish this,laser head 12 is provided with a plurality of threaded apertures, suchas rivet nuts 88 or the equivalent, at suitable angular intervals nearthe left end thereof and near the right end thereof. (In practice, fourriveted nuts are used at each end.) During assembly of the laser head,before elastomer pads 54 are applied, respective support screws (notshown) are individually inserted in each of rivet nuts 88 and arebrought into contact with a corresponding point of capillary tube 14 tothereby support laser tube 10 withinlaser head 12. Laser tube 10 is nowoperated and the laser output obtained therefrom is measured. Then, thesupport screws are adjusted until the laser output is a maximum and issubstantially independent of the angular position of the laser headcylinder 12. While laser tube 10 is maintained in this desired positionby the support screws, a suitable amount of elastomer, in un- I curedliquid form, is applied through appropriately placed holes in the wallof laser head 12 to provide each of the plurality of elastomer pads 54connecting bulb 26 of laser tube 10 to the inner surface of laser head12, as shown in FIGS. 1 and 2. Although the support screws may bepermanently left in their adjusted position, usually they are removedfrom rivet nuts 88 after the elastomer of pads 54 has fully cured. It isfor this reason that they are not shown in FIG. 1. In the case where thewalls of the container are sufficiently thick, the aperture walls may bethreaded, and rivet nuts dispensed with.

The optimum positioning of laser tube within laser head, just described,is'useless unless laser tube 10 is both mechanically and thermallystable, so that the initial peaking of the laser output is maintainedover a long period of time. Otherwise, thermal expansion or contraction,resulting from changes in temperature, and slight shifting in positiondue to vibration or shock, both of which cannot be avoided, will cause aslight change in the spatial relationship between mirrors 22 and 24 ofthe laser tube optical cavity.

Mechanical and thermal stability is provided in the laser device ofFIGS. 1 and 2 by employing thickwalled single capillary tube 14, towhich both mirrors 22 and 24 are attached, and then supporting thiscapillary tube at two relatively widely spaced points by left endportion 28 and right end portion 30 of bulb 26. This supplies therequired mechanical and thermal stability to maintain mirrors 22 and 24in strict parallel relationship once that relationship has beeninitially set.

The output of the laser radiation, in addition to being dependent uponthe parallel relationship between mirrors 22 and 24, as just discussed,is also a function of the temperature of the gas discharge in bore 16 oflaser tube 10. Thick-walled capillary tube 14 itself supplies a gooddeal of thermal insulation for bore 16. However, in addition thereto,the arrangement of cathode 16 and gas-filled bulb 26 acts as a bottle tomaintain a relatively large portion of bore 16 at a uniform temperature.In particular, this bottle effect results from the fact that the gaspressure in laser 10 is only a few Torr, (such as 3 Torr) which is inthe viscous flow range, causing the enclosed laser gas surrounding boretube 14 to circulate, thereby maintaining the bore temperature uniform.Therefore, under operating conditions, the temperature of the gasdischarge in bore 16 is relatively independent of fluctuations inambient temperature surrounding laser tube 10.

What is claimed is:

l. A laser discharge device comprising a single axial bore tube of givenlength having a bore therethrough, a first mirror terminating andsealing said bore at one end of said tube, a second mirror terminatingand sealing said bore at the other end of said tube, an anode electrodesituated within said bore at a location towards said one end of saidtube, a bulb surrounding said tube and attached thereto only at firstand second ends of said bulb, said bulb having said first end bonded tothe outside of said tube over a first region thereof locatedintermediate said anode electrode and said second end of said tube, saidbulb having said second end bonded to the outside of said tube over asecond region thereof located intermediate said first end of said bulband said second end of said tube, a cathode electrode situated outsideof said tube but inside of said bulb, said tube having a hole throughthe wall thereof communicating between the inside of said bulb and saidbore, and a laser gas filling said bulb and bore, said laser gas beingresponsive to a discharge therethrough extending from said anodeelectrode through said bore and hole to said cathode electrode forcreating a population inversion therein and for stimulating laseremission therefrom in said bore.

2. The laser discharge tube defined in claim 1, wherein said bore tubeis a thick-walled capillary tube.

3. The laser defined in claim 1, wherein said first and second regionsof said tube to which said bulb is bonded are spaced from each other bya distance which is large relative to the dimensions of said regions,and, wherein said bulb contacts said tube only at said first and secondregions.

4. The laser defined in claim 1, wherein said hole is located towardsaid second end of said bulb.

5. The laser defined in claim 1, wherein said bulb is oriented insubstantially coaxial relationship with said bore tube.

6. The laser defined in claim 5, wherein said cathode electrode is acylindrical cold cathode oriented in spaced substantially coaxialrelationship with said bore tube.

1. A laser discharge device comprising a single axial bore tube of givenlength having a bore therethrough, a first mirror terminating andsealing said bore at one end of said tube, a second mirror terminatingand sealing said bore at the other end of said tube, an anode electrodesituated within said bore at a location towards said one end of saidtube, a bulb surrounding said tube and attached thereto only at firstand second ends of said bulb, said bulb having said first end bonded tothe outside of said tube over a first region thereof locatedintermediate said anode electrode and said second end of said tube, saidbulb having said second end bonded to the outside of said tube over asecond region thereof located intermediate said first end of said bulband said second end of said tube, a cathode electrode situated outsideof said tube but inside of said bulb, said tube having a hole throughthe wall thereof communicating between the inside of said bulb and saidbore, and a laser gas filling said bulb and bore, said laser gas beingresponsive to a discharge therethrough extending from said anodeelectrode through said bore and hole to said cathode electrode forcreating a population inversion therein and for stimulating laseremission therefrom in said bore.
 2. The laser discharge tube defined inclaim 1, wherein said bore tube is a thick-walled capillary tube.
 3. Thelaser defined in claim 1, wherein said first and second regions of saidtube to which said bulb is bonded are spaced from each other by adistance which is large relative to the dimensions of said regions, and,wherein said bulb contacts said tube only at said first and secondregions.
 4. The laser defined in claim 1, wherein said hole is locatedtoward said second end of said bulb.
 5. The laser defined in claim 1,wherein said bulb is oriented in substantially coaxial relationship withsaid bore tube.
 6. The laser defined in claim 5, wherein said cathodeelectrode is a cylindrical cold cathode oriented in spaced substantiallycoaxial relationship with said bore tube.